The field of the invention is large excavators of the type which are shipped in sections and assembled on the job site, and particularly, to the fabrication and assembly of the swing gear and swing bearing assembly on such machines.
The base, or tub, on large excavators supports a revolving frame upon which the operator's cab, machinery and boom are mounted. The weight of the revolving frame is supported by a swing bearing assembly which is comprised of a series of ball or roller bearings disposed on the base in a circle about a central pintle. The revolving frame is swung on the swing bearing assembly by a swing drive unit which engages a circular swing gear that mounts to the base radially inward from the swing bearing.
To provide a smooth swinging motion and to evenly distribute the load among the swing bearings, the swing bearing assembly is disposed between upper and lower bearing pads which are attached respectively to the revolving frame and base and which are machined flat to provide uniform spacing therebetween. A variation in spacing of 0.001 inches over a distance of one foot can result in a doubling of the load on certain bearings and a sliding or skipping action in others. Such uneven loading results in increased wear and the sliding or skipping causes a rough swing motion.
Although the bearing pads on the revolving frame are machined flat in the plant, the large size of the base and revolving frame (35 to 100 feet) requires that they be shipped to the job site in sections where they are reassembled. Distortion of the machined surfaces on the bearing pads typically occurs from this reassembly process, primarily as a result of the heat generated during welding operations. As a consequence, it has been common practice for a number of years to machine the lower bearing pad surface after the base has been reassembled at the job site, but no satisfactory method of remachining the upper bearing pad surface has been found.